1. Field of the Invention
The present invention relates to the field of electric power cables, in particular for submarine use.
2. Description Of Related Art
Power cables commonly comprise a sheath layer consisting of lead to be used as a radial water barrier. This relates to submarine power cables, but is also relevant for other cables subjected to potential humid environment. Water and humidity are detrimental to electrical insulating materials for all power cables conducting electricity at medium and high voltages.
Electric cables are known without a lead sheath, but with other layers for preventing penetration of water or moisture into the insulation.
The German Patent DE4135634 relates to an optical communication cable with a metallized coating on the outside of the cable as vapour barrier.
The British patent GB1579123 relates to a submarine coaxial cable whereby a sealed return coaxial conductor also acts as a moisture barrier against moisture ingress to a dielectrical material.
The Japanese patent application JP61165907 relates to a submarine power cable with a laminated layer comprising a metallic foil. This patent is about a radial conductor barrier replacing a longitudinal filling compound.
None of these patents and application relates to high voltage power cables.
It is of particular importance for medium or high voltage electric power cables including insulation, to keep the insulation in a relatively dry environment. When high electrical stress is present in combination with humidity as in a medium or high voltage submarine cable, the insulation is exposed to a degradation process, which is often called water treeing. This degradation process increases with the level of humidity present. Medium and high voltage extruded polymer power cables are usually intended to have a calculated life expectancy of the insulation system. The degradation process should be so slow that it does not reduce the life expectancy of the cable, typically 25 to 50 years.
The problems with humid environments are in prior art solved by enclosing the insulation in a lead sheath as it is impervious to water, it tolerates bending without buckling, a degree of cyclic mechanical stresses and so on.
Because lead has a long history of successful operational experience, its use as water barrier has not been seriously challenged.
There are also some serious drawbacks when using a lead sheath as water barrier:                It is heavy, making transport, laying and handling of the cable requiring the use of heavy duty equipment        It has relatively low electrical conductivity, so when high short circuit rating is required, additional conductivity has to be added        For dynamic use, the lead sheathing results in the cable having a low fatigue life making it unsuitable.        
One of the problems that may arise when using possible alternatives to lead sheath is that the alternatives may represent water barriers that are less moisture proof than lead.
In medium and high voltage cables, a hygroscopic barrier that is reliable, throughout the lifetime of the cable, is of vital importance. Onset of insulation degradation, often called water treeing, will occur when the humidity at the insulation interface is high, i.e. around 70% relative humidity (RH).
A required barrier that prevents moisture to build to such level at the insulation has up to now been solved by a lead sheath layer with its advantages and drawbacks.